1. Field of the Invention
The present invention relates to the respiratory care of a patient and, more particularly, to a ventilator that monitors the pressure of the breathing gas supplied to and exhaled from the patient, and controls the pressure and/or flow rate of the breathing gas supplied by the ventilator to the patient to nullify the work of breathing imposed on the patient by the breathing apparatus.
2. Prior Art
Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient""s breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient""s ability to breath independently.
A patient whose breathing is being supported by a ventilator typically receives breathing gas through a ventilator conduit. The ventilator conduit generally consists of two flexible conduits, an inhalation conduit and an exhalation conduit, connected to a wye fitting. The free ends of the conduits are attached to the ventilator so that the inhalation conduit receives breathing gas from the ventilator""s pneumatic system and the exhalation conduit returns gas exhaled by the patient to the ventilator. The wye fitting is typically connected to the patient""s breathing attachment, which is oftentimes an endotracheal tube, which conducts breathing gas into the lungs of the patient, and exhaled gas from the lungs of the patient to the exhalation conduit.
In those instances where a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination. These modes can be defined in three broad categories: spontaneous, assisted or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure, and then the ventilator xe2x80x9cassistsxe2x80x9d the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath.
During spontaneous or assisted ventilation, the patient is required to xe2x80x9cworkxe2x80x9d (to varying degrees) by using the respiratory muscles in order to breath. The work of breathing can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient efforts to breath by decreasing the work of breathing to sustain the breath. Still other techniques have been developed that aid in the reduction of the patient""s inspiratory work required to trigger a ventilator system xe2x80x9cONxe2x80x9d to assist the patient""s breathing. It is desirable to reduce the effort expended by the patient in each of these phases, since a high work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients.
The work of breathing of a patient breathing spontaneously while intubated and attached to the ventilator during ventilatory support by may be divided into two components: first, the imposed work of breathing of the breathing apparatus; and second, the physiologic work of breathing of the patient. The imposed work of breathing is the resistive work of breathing imposed by the breathing apparatus (the physical construct of the entire ventilation support external to the patient"" lungs, i.e., the endotracheal tube, the ventilator conduit, the medical ventilator, etc.) upon the spontaneously breathing patient receiving ventilator support. The physiologic work of breathing of the patient consists of two components: first, the resistive work of breathing of the airways of the patient, and two, the elastic work of breathing of the lungs and the chest wall. It is desirable to reduce or, even more desirable, to nullify the imposed work of breathing as the patient may be detrimentally affected by an excessively high expenditure of energy early in the inspiration process caused by the respiratory muscle force required to overcome the imposed work of breathing of the breathing apparatus. Patient""s may fatigue under the imposed work of breathing workload which predisposes the patient to respiratory muscle fatigue, respiratory distress, respiratory or ventilator dependancy, and/or failure. Nullification of the imposed work of breathing also allows for the contemporaneous determination of the physiologic work of breathing.
The conventional methods and apparatus for reducing or minimizing the imposed work of breathing are inadequate. Typically, these conventional efforts rely upon a means of xe2x80x9ctriggeringxe2x80x9d the ventilator to supply inspiratory ventilation support upon the sensing of an inspiration effort. The conventional means for triggering the ventilator may be classified as either pressure or flow-by triggering. In conventional pressure triggering, the withdrawal of the small volume of gas that occurs as a breath is initiated by the patient results in a corresponding drop in pressure which is monitored via a pressure sensor that is typically disposed within the ventilator conduit at or near the wye piece or within either the inhalation conduit or the exhalation conduit. At the onset of spontaneous inhalation by the patient, the pressure change is detected in the breathing circuit which functions to trigger the ventilator xe2x80x9cONxe2x80x9d to then actively inflate the lungs of the patient during ventilation support. Several disadvantages are associated with the use of conventional pressure triggering to reduce the imposed work of breathing. First, the chosen pressure measurement sites produce a pressure signal that measures the pressure of the breathing gas proximate the sensors which is remote from the actual intratracheal pressure drop occurring within the patient""s trachea throughout the spontaneous inhalation effort. The measured sensors are then used as a basis for regulating or controlling the amount of pressure or flow rate (to generate the requisite pressure) of breathing gas applied to the lungs. Because the chosen sites are so remote from the lungs of the patient, the resulting pressure measurements are an inherently inaccurate measurement of the pressure on the airways and lungs of the patient which causes a marked increase in the effort or work to inhale by the patient as the regulated amount of breathing gas applied to the patient is calculated in error due to the xe2x80x9capproximatedxe2x80x9d value of the pressure drop sensed.
Second, and once again because of the remote pressure sensing measurement sites, in conventional pressure triggering there is a significant amount of lag time and associated negative pressure that always occurs between the onset of the patient""s inspiratory effort and the time that the gas pressure or flow reaches the patient""s airway. This lag time is generally referred to as a ventilator""s response time, and commonly occupies a small but significant portion of a patient""s total inspiration time. The pressure waves that are indicative of the pressure drop travel to the pressure sensor at the speed of sound in the breathing gas, which is approximately 1 millisecond per foot. Due to factors inherent in conventional ventilator design and the prior art locations of the pressure sensing site, the resulting patient inspiration effort can typically continue for as long as 40 to 500 milliseconds without ventilator assistance. Thus, under the conventional pressure drop triggering schemes, the pressure drop, which a patient is required to create in order to trigger a breath in a closed breathing circuit, can require a significant expenditure of energy by the patient. This imposed work of breathing on the patient can be detrimental in that respiratory muscles already loaded and nearly fatigued by an operation or other patient conditions may continue to fatigue, which, if this process continues, may result in the failure or severe compromise of the ventilation support procedure. Additionally, the forced respiratory work required to trigger ventilation may be beyond the capacity of infant, small children, or patients"" suffering from trauma or disease.
In flow-by triggering, the signal to cycle xe2x80x9cONxe2x80x9d the ventilator to deliver pressure or flow support of a patient""s inhalation effort is determined by monitoring the flow in the patient""s ventilator conduit or inside the ventilator. In such a system, a single flow sensor is typically positioned inside the ventilator to monitor the flow of gas that a patient withdraws from the ventilator system via the ventilator conduit and triggers a pressure or flow based breath support when the patient""s inspiratory flow equals a certain level. However, such a closed system flow based trigger is not an improvement over a conventional pressure triggering system, because all of the same delays and work required of the patient (imposed on the patient) are still present. In addition, a significant negative pressure drop is still required to start the breath and there is no continuous flow to support the earliest phase of the breath. Significantly, even if there were some form of continuous flow to support the earliest phase of the breath in an effort to minimize the imposed work of breathing required of the patient to cause the requisite negative pressure drop, the remote location of the flow sensor would still cause inappropriate application of the pressure or flow based breath support due to the inherent inadequacy of the measurement site. Therefore, in conventional triggering means developed to minimize the imposed work of breathing of the patient, the patient must still overcome the substantial resistance and inertia of the breath triggering process. The present invention overcomes the prior art limitations, and due to the diminimus response time, aids in reducing and effectively nullifying the imposed work of breathing of the patient.
Optimal ventilatory assistance requires a balance between appropriately minimizing physiologic workloads to a tolerable level and decreasing imposed resistive workloads to zero. This balance should insure that the patient is neither overstressed nor oversupported. Insufficient ventilatory support places unnecessary demands upon the patient""s compromised respiratory system, thereby inducing or increasing respiratory muscle fatigue. Excessive ventilatory support places the patient at risk for pulmonary-barotrauma and other complications of mechanical ventilation.
From the above, it is clear that it would be desirable to have a medical ventilator that reduces, and effectively nullifies, the patient""s imposed work of breathing. Further, it is clear that it would be desirable to have a medical ventilator that can supply a clinician""s desired mode or technique of spontaneous or assisted ventilation support while contemporaneously nullifying the imposed work of breathing of the patient. Such a medical ventilator is unavailable in current systems.
An excessively high expenditure of energy (work of breathing) by the patient, early in the inspiratory process, can be detrimental to the patient. Patients may fatigue under these workloads, leading to further respiratory distress and/or failure. The required energy expenditure can also create difficulties in weaning the patient from the ventilator, leading to patients who become ventilator dependent. Thus, reducing the energy expenditure of the patient to an appropriate level and minimizing or eliminating unnecessary energy expenditures while breathing on a mechanical ventilator is advantageous for the patient.
Few improvements have been made in the reduction of the work of breathing imposed by the ventilation breathing apparatus (the physical construct of the entire ventilation support system external to the patient""s lungs, i.e., the endotracheal tube, the ventilator conduit, the medical ventilator, etc.). The conventional methods and apparatus of prior art ventilators, or ventilator systems, for reducing or minimizing the imposed work of breathing are inadequate and require the patient to unnecessarily expend a significant amount of energy in an effort to overcome the imposed work of breathing to initiate the desired ventilation support. This imposed work of breathing on the patient can be detrimental in that the patient""s respiratory muscles, which may be already loaded and nearly fatigued by an operation or other patient conditions, may continue to fatigue and, if this process continues, may lead in the failure or severe compromise of the ventilation support procedure. The principle object of the present invention is to provide a method and corresponding apparatus for nullifying the work of breathing imposed by the ventilation breathing apparatus by continually modulating the pressure and/or flow rate of the breathing gas supplied by the ventilator to maintain the pressure of the breathing gas near the distal end of a breathing attachment, such as the preferred endotracheal tube, at a constant, predetermined, baseline pressure throughout an inhalation effort of the patient.
A further object of the present invention is to provide a method and corresponding apparatus that may be used simultaneously with pressure support ventilation methods and modes known to those skilled in the arts to improve patient efforts to breath by both reducing the work of breathing required to sustain a breath and for nullifying the work of breathing imposed by the ventilation breathing apparatus.
Briefly, the present invention is directed to a medical ventilator for supplying a breathing gas to a patient for use in a medical procedure, such as ventilation support. The breathing gas being received into the medical ventilator from a gas source of one or more breathing gases and the gas exiting the ventilator being in flow communication with a functionally open ventilator conduit. The ventilator conduit has an endotracheal tube in fluid communication with the lungs of the patient. A pressure sensor is disposed near the distal end of the endotracheal tube to sense the pressure of the breathing gas proximate the pressure sensor which is indicative of the relative intratracheal pressure of the patient. A monitoring means, such as a microprocessor, is connected to the pressure sensor to monitor the pressure, to detect when the pressure sensed deviates from a predetermined baseline pressure, and, if the pressure of the gas proximate the pressure sensor is below the predetermined baseline pressure, to generate a pressure response signal thereof, and, if the pressure of the gas proximate the pressure sensor is above the predetermined baseline pressure, to generate a termination signal thereof.
The medical ventilator also has a gas delivery means that is in fluid/flow communication with the gas source for receiving the breathing gas from the gas source. The gas delivery means regulates the pressure and/or flow rate of the breathing gas supplied to the patient. Further, the medical ventilator has a regulating means operatively coupled to the gas delivery means and the monitoring means for pressure and/or flow rate controlling the breathing gas supplied to the patient so that the pressure of the breathing gas near the distal end of the endotracheal tube is maintained at the predetermined baseline pressure. The gas delivery means comprises a pneumatic system, having at least one actuator, responsive to the monitoring means via the regulating means, for controlling the pressure and/or the flow rate of the breathing gas so that the pressure of the breathing gas near the distal end of the endotracheal tube is maintained at the predetermined baseline pressure.
The regulating means, responsive to the pressure response signal from the monitoring means that indicates that the pressure of the gas proximate the pressure sensor is below the predetermined baseline pressure, may adjust at least one of the actuators of the gas delivery means, as necessary, to increase the pressure and/or flow rate of the breathing gas delivered by the medical ventilator so that the pressure of the pressure of the gas proximate the distal end of the endotracheal tube is maintained at the predetermined baseline level. Similarly, the regulating means, responsive to the termination signal from the monitoring means that indicates that the pressure of the gas proximate the pressure sensor is above the predetermined baseline pressure, may adjust at least one of the actuators so that the pressure of the pressure of the gas proximate the distal end of the endotracheal tube is maintained at the predetermined baseline level. Preferably, the regulating means is automatically and proportionally controlled by the monitoring means.
Moreover, the present invention relates to a method of controlling, for any selected period of time, a medical ventilator providing some form of ventilation support to a patient supplied with a breathing gas from the medical ventilator for nullifying the imposed work of breathing during the ventilation of the patient. The gas being pressure and/or flow rate controlled by the ventilator, the method comprising the steps of: delivering the breathing gas from the ventilator into the lungs of the patient via a ventilator conduit having an endotracheal tube; sensing the pressure of the breathing gas within the endotracheal tube; monitoring the sensed pressure to determine when the sensed pressure deviates from a predetermined baseline pressure; regulating the breathing gas supplied by the ventilator when it is determined that the sensed pressure is less than the predetermined baseline pressure; and restoring the breathing gas supplied by the ventilator when it is determined that the sensed pressure is greater than the predetermined baseline pressure. The method may also include the further step of determining the patient physiologic work of breathing.
The above and other objects and advantages of the present invention will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.